This application claims priority to U.S. Provisional Patent Application Ser. No. 60/453,068, filed Mar. 7, 2003, the entire content of which is hereby incorporated by reference.
This invention generally relates to photosensitizers for titanium dioxide having transition metal complexes on the surface of the titanium dioxide and their solid state methods of synthesis.
The use of sensitized semi-conductors for the direct conversion of sunlight into electricity is an attractive alternative to more traditional fossil fuel based energy production. A photovoltaic cell called the Grätzel cell, after inventor Michael Grätzel, has become the industry standard to date. This cell uses a Ru(II) due, involving the anchor ligand 4,4′-dicarboxy-2,2′-bipyridine, to sensitize titanium dioxide (TiO2). With an efficiency of only about 10% and a fairly high cost due to the transition metal catalyst Ru(II), the dye-sensitizer solar cell has not yet found widespread commercial utility.
The principle set-up and operation of the dye-sensitized solar cell begins with light absorption (such as light energy from the sun) by a dye chemically absorbed on a semiconductor coated on the surface of a glass electrode. The semiconductor film is in contact with an iodide/triiodide redox electrolyte. A glass counter electrode plate is situated over the semiconductor and the edges of the two glass plates are sealed. After the absorption of a photon of light, the dye, typically a transition metal complex whose molecular properties are specifically engineered for the task, is able to transfer an electron to the semiconductor. The electric potential developed inside the bulk material causes the flow of electrons through the semiconductor film to a conducting glass electrode. After passing through an external circuit and having delivered power to a load, the electrons return to the cell at the counter electrode. Triiodide is then converted to iodide which reduces the photo-oxidized dye to its original ground state. Ru(II) is the metal chromophore present in the dye-sensitized solar cells used today.
Current solar cell technology involves the absorption or doping of transition metal complexes as sensitizers onto a nanocrystalline semiconductor such as titanium dioxide (TiO2). Titanium dioxide is a semiconductor that is useful in the conversion of visible and ultraviolet radiation into electricity. Such conversion depends upon the injection of photoexcited electrons into the conduction band of the titanium dioxide. This photoinjection is accomplished by a sensitizer or dye, which absorbs incident radiation, thereby becoming a high energy, photoexcited material. This photoexcited material can then inject an electron, or sensitize, the titanium dioxide, thereby rendering it as conducting and creating an electric current.
A critical component of such a photosensitized device is the sensitizer or dye, because it determines the efficiency of electron injection into the titanium dioxide and thus the efficiency of the device (Nazeeruddin et al., 1993; Bignozzi et al., 2000; Schwarz et al., 2000). Sensitizers or dyes are typically deeply colored materials that absorb light in the visible and ultraviolet region of the spectrum. They are traditionally prepared independent of the titanium dioxide by a series of synthetic steps involving both organic and inorganic chemical methodology (Nazeeruddin et al., 2001). These preparations involve the actual synthesis, followed by purification or isolation of the product, followed by identification or characterization of this product. Purification and characterization may occur at each and every step of the traditional synthetic method. The prepared sensitizer is then applied to the surface of the titanium dioxide to provide the sensitized semiconductor. The efficiency of charge injection is measured by a variety of techniques. Numerous sensitizers have been prepared and tested according to this protocol (Hagfeldt et al., 2000; Kalyanasundaram et al., 1998; Kelly et al., 2001).
The vast majority of previously prepared sensitizers have been complexes of Ru(II) (Beley et al., 2000; Hara et al., J. Photochemistry and Photobiology 2001, Langmuir 2001, and J. Phys. Chem. B 2002; Islam et al., 2001; and Yanagida et al., 2002). The limiting factor in the testing of many different sensitizers is the synthesis of the sensitizer.
What is needed, therefore, is a faster and more efficient method for synthesizing transition metal complexes which may serve as sensitizers for titanium dioxide.